A well-known packaging configuration for microcirucits is known as a flat package or flat pack, which comprises a rectangular metal box having a base and side walls with leads passing through openings in one or more of the side walls by way of glass-to-metal seals. A flat pack of known form is of layered or sandwich construction in which a glass ring is provided over a metal baseplate, with one or more lead frames installed on the glass ring and over which a second glass ring is installed with a metal frame thereover. This layered structure is thermally processed to fuse the leads of the lead frames within the glass rings and to seal the glass rings to the underlying baseplate and overlying metal frame. A metal cover plate can be bonded to the metal frame after installation of a microcircuit within the package. Such packages are widely employed in the microcircuit industry but are not suitable for conditions of high stress such as found in heavy industrial, military, and aerospace applications. Under high stress forces, the glass-to-metal seals between the metal frame or the baseplate can fail, resulting in loss of the hermetic package seal, with resultant contamination of the microcircuit contained within the package and usually failure of the circuit.
Another known metal flat package comprises an open-topped metal box of rectangular configuration and in which each lead extends through a respective opening in a side wall and is sealed therein by a glass-to-metal bead seal. The edge of the side walls of this all-metal package can be made very flat to assure good solder sealing or welding of a cover thereon to complete the package enclosure. This type of flat package operates well under most conditions, but is relatively expensive to fabricate.
Another package of known configuration is shown in U.S. Pat. No. 4,262,300, assigned to the assignee of this invention, and in which the side walls are composed of separate components which are secured together to form a continuous side wall frame and onto which is secured a one-piece continuous top frame member having a uniform upper surface to which a package lid can be bonded.